The present invention relates to a multi-beam laser exposure suitable for an image forming apparatus such as a color printer apparatus, a high speed copying machine and a color copying machine, each having a plurality of drums.
For example, an image forming apparatus such as a color printer, a color copying machine, or the like, each having a plurality of drums, uses a plurality of image forming units for forming images corresponding to color-separated color components, and a laser exposure unit for providing a plurality of image data, i.e., a plurality of laser beams in units of color components to these image forming units. As the laser exposure unit, for example, a plurality cf laser exposure units are arranged, or a multi-beam laser exposure unit which can produce a plurality of laser beams is arranged.
The multi-beam laser exposure has a semiconductor laser element serving as a light source, a first lens group for focusing the beam size of a laser beam emitted by the laser element to a predetermined size, a light deflection unit for continuously reflecting the laser beam focused by the first lens group in a direction perpendicular to the feed direction of a recording medium, a second lens group for imaging the laser beam deflected by the light deflection unit at a predetermined position of the recording medium, and the like. In general, the direction in which the laser beam is deflected by the light deflection unit is called a main scanning direction, and the direction in which the recording medium is rotated, i.e., the direction, perpendicular to the main scanning direction is called a sub-scanning direction.
As a light scanning unit of this type, for example, depending on the image forming apparatus to which the light scanning unit is applied, a plurality of light scanning units are arranged in correspondence with the image forming units or a multi-beam light scanning unit which can produce a plurality of laser beams is arranged.
If image information can be recorded on a recording medium using, e.g., N laser beams, the rotational speed of a rotary mirror and the image frequency can be reduced to 1/N.
By arranging M groups of light sources including N laser beams in correspondence with the number of color-separated color components, a compact light scanning unit suitable for an image forming apparatus that can form a color image can be provided.
However, in order to guide M groups of laser beams to the light deflection unit in a state wherein they can be considered as a single laser beam, the M groups of laser beams must be synthesized on the light source side of the light deflection unit. In this case, a sufficiently large distance must be assured between the light deflection unit and the light source, or the laser beams incident on the reflection surface of the light deflection unit must be incident to be separated from each other in the direction perpendicular to the rotation direction of the reflection surface, i.e., the sub-scanning direction.
When a large distance is assured between the light source and the light deflection unit, the light scanning unit becomes large in size. On the other hand, when the laser beams are incident on the reflection surface to be separated from each other in the sub-scanning direction, the imaging characteristics may deteriorate, or the bend amount differences in the scanning direction in units of M groups of laser beams, variations in sectional beam size on the image surface upon changes in refractive index of the lens material caused by environmental changes in units of M groups of laser beams, and the like may increase.
If the distance between the final lens surface and the image surface is increased for the purpose of attaining a size reduction of the light scanning unit, the bend amount differences in the scanning direction in units of M groups of laser beams increase. In this case, the bend amount can be reduced by increasing the distance between the reflection surface of the light deflection unit and the final lens surface, but the size of the light scanning unit increases. On the other hand, when the distance between the reflection surface and the final lens surface is increased, the driving frequency upon driving the laser element of each light source, i.e., the image(frequency must be raised since the effective scanning angle decreases. As a consequence, cost increases in terms of noise measures and the frequency characteristics of the driving unit.
Furthermore, upon optimizing the lens passing positions of the M groups of laser beams separated in the sub-scanning direction to obtain uniform optical characteristics to be given to these laser beams, the laser beams that pass through positions offset in the sub-scanning direction from the optical axis of the system of the light scanning unit may suffer coma different from that for the remaining laser beams.
When each of the M groups of laser beams includes N laser beams, a plurality of half mirrors as semi-transparent mirrors are used to synthesize the N laser beams so that they are substantially considered as a single laser beam. In this case, when the laser beams pass through different numbers of half mirrors, the light amount difference, spherical aberration difference, coma difference, and the like among the laser beams increase, resulting in different beam sizes.
Furthermore, when each of the M groups of laser beams includes N laser beams, since each group of laser beams has a width in the sub-scanning direction, the tilt, in the main scanning direction, between the exposure start and end positions of laser beams scanned in a single scan may increase up to a visible level.
Moreover, since each of the M groups of laser beams includes N laser beams, the optical energy on the image surface may vary due to the phase difference or wavelength variations of the laser beams when the light intensities of all the laser beams that have reached the image surface are synthesized. When variations in optical energy due to the phase difference or wavelength variations have exceeded a predetermined amount, the image may be locally lost or toner supply to an unexposed portion may be lost when such unit is built in the image forming apparatus.